The field relates to structural material that immobilizes heavy metals impacting thereon, thus providing a safe environment for personnel in the area while preventing subsequent environmental degradation. More particularly, provided in one embodiment is a fiber reinforced foamed-concrete structure that includes constituents, including aluminum, that significantly reduce the mobility of heavy metals, including fine particles thereof, when the structure is subjected to various forms of erosion, including that from acidic aqueous solutions.
In order to train military personnel, live-fire training villages have been employed in training exercises. During these exercises, projectiles are entrained in the walls (panels) simulating the buildings. Projectile-absorbing materials, such as rubber blocks or rubber-lined wooden boxes filled with quartz sand, provide only for absorbing the projectiles, i.e., they are single purpose materials. For example, they do not have the ability to control erosion of the projectile material, e.g., through boosting the alkalinity of water that may contact the spent projectiles. Unless they are collected shortly after dispensing, heavy metals; (to include all such heavy metals defined by the Environmental Protection Agency (EPA) as being harmful to the environment) entrained in these structures corrode and hazardous material is released. Particular heavy metals of interest include lead, arsenic (III), barium, bismuth, cadmium, chromium (II), copper, iron, nickel, selenium, and zinc.
Hazardous wastes containing excessive amounts of leachable heavy metals are banned from land disposal. For example, the current regulatory threshold limit for lead under the Resource Conservation and Recovery Act (RCRA) is 5 mg/l as measured by the Toxic Characteristic Leaching Procedure (TCLP), United States Environmental Protection Agency (USEPA) method 1311 (SW-846). The test is aimed at identifying the tendency of wastes to generate a leachate with concentrations of contaminants greater than the values listed at Volume 40, Code of Federal Regulations, Part 261.24, page 406, revised Jul. 1, 1988. Waste materials containing TCLP lead levels in excess of 5 mg/l are defined as lead-toxic hazardous waste and restricted from land-filling under current regulations. The cost of disposing lead toxic hazardous waste materials is in excess of $200.00 per ton plus the cost of transportation to the distant sites.
Patents dealing with the addition of phosphates to wastes or contaminated soils to reduce the solubility of lead and its compounds include U.S. Pat. No. 5,202,033, In Situ Method for Decreasing Heavy Metal Leaching from Soil or Waste, issued to Stanforth et al., Apr. 13, 1993 that describes a method of adding a combination of additives and buffering agents such as phosphates, carbonates, and sulfates. Other patents, such as U.S. Pat. No. 5,637, 355, Method of Nonhazardous Removal of Paint Containing Heavy Metals and Coating Preparation for Performing the Same, issued to Stanforth et al., Jun. 10, 1997 detail methods for applying a coating of phosphates and sulfides over a lead-based paint to make it insoluble for subsequent safe removal of the paint from a structure.
Because there is significant concern over the release of heavy metals into the environment, structure components have been developed which inhibit mobility of the heavy metals, preventing their leaching outside the components. Recently a foamed, fiber-reinforced concrete with selected additives has been used to fabricate these structures and reduce mobility of the heavy metals entrained therein. See U.S. Pat. No. 6,264,735 B1, Low-Lead Leaching Foamed Concrete Bullet Barrier, issued to Bean et al., Jul. 24, 2001 (hereafter the ""735 patent), and incorporated herein by reference. This type of concrete has been particularly successful in structural panels for live-fire training villages, enclosures used for grenade training, and backstops on firing ranges. Foamed-concrete is the best type of heavy metal projectile-trapping material because it does not burn like wood or rubber, or rot, like wood, and does not erode like sand or soil. Chunks of heavy metals are more firmly entrained in this material than in wood or rubber. Finally, it is alkaline and thus suited to resist erosion caused by naturally occurring aqueous acidic flows.
When projectiles formed of heavy metals impact foamed concrete structures, the projectiles and their fragments are entrained therein. Any moisture in the concrete structure can initiate a chemical reaction on the surface of the heavy metals, causing corrosion and, eventually, erosion, or leaching, of the surface material. With the addition of water, the alkalinity of even the modified foamed-concrete decreases somewhat and the pH of the water in contact with the bullet fragments may be lowered enough to cause the heavy metals and their corrosion products to dissolve. Additionally, fine heavy metal particles may be entrained in concrete fragments that spall off the structures, falling to the ground. In a moist environment, the heavy metal fragments in these distributed spalled materials are subject to corrosion and dissolution, with subsequent leaching into the environment. In some scenarios, it may be severe enough to curtail training on firing ranges. Thus, there is a need in the art for an improved structure that robustly prevents mobility of heavy metals entrained in it, thus significantly reducing or eliminating the risk of soil and groundwater contamination.
Advantages of a preferred embodiment of the present invention include:
dense projectiles will not rebound or ricochet from it;
structures constructed of it are insect proof;
low maintenance;
reduces the occurrence of dust having heavy metals entrained therein;
heavy metals later entrained in it chemically react with the material and any moisture that may contact the material or the heavy metal, immobilizing it;
safe, inexpensive, common construction materials are used as components;
only standard construction skills are needed to fabricate;
at the end of its useful life, the material may be deposited in a standard landfill; and
the material is non-flammable when impacted by tracer or incendiary projectiles.
These advantages are realized in a preferred embodiment of the present invention envisioned in a structural component made from Portland cement-based, foamed, fiber-reinforced concrete that uses crushed limestone as the fine aggregate component to act as a buffer and contains materials, such as calcium phosphate and an aluminum compound, that react with heavy metals to produce a passivating (mobility degrading) coating on the heavy metals. Further, the coating establishes a sticky gel coating that also entrains any fine particles that may have spalled from the heavy metal or metal xe2x80x9csmearedxe2x80x9d on fine particles of the material that may have spalled from the material""s surface.
When stabilizing entrained small arms bullets comprising a lead core and a copper jacket, adding aluminum to the material described in the ""735 patent facilitates formation of complex lead and copper aluminum phosphate compounds within the structure that are less reactive and less soluble than the lead or copper phosphates and carbonates produced by the formulation of the ""735 patent. One compound that forms is hydrated aluminum hydroxyphosphate, PbAl3(PO4)2(OH)5 monohydrate. As it naturally occurs, this mineral is plumbogummite. This and related lead phosphates (pyromorphites) have such low solubility that water samples taken from roadside soils that contain aluminum-rich clays that are contaminated from recent past use of leaded fuels typically contain less than 1 ppm of lead.
Also provided is a method of producing, foamed, fiber-reinforced concrete for forming structures that later entrain projectiles formed of heavy metals and essentially restrict mobility of these heavy metals, in all forms, to the structure itself. The method provides for dry mixing Portland cement with a suspending agent, such as a methylcellulose compound, to form a dry mixture. Water is mixed with a fine aggregate to form an aqueous mixture. The dry mixture of cement and suspending agent is mixed into the aqueous mixture to form a slurry. Powdered calcium phosphate and an aluminum compound are mixed into the slurry. Mixing is continued until all constituents are thoroughly distributed throughout the resulting mixture. The density of the resulting mixture is determined by removing a known volume of the mixture and determining its mass. A pre-formed, water-based foam is added to the resulting mixture until the density of the resulting mixture is adjusted to meet a pre-specified design density. Pre-specified fibers are mixed in after the density has been adjusted. Mixing is continued until the fibers are distributed evenly throughout. The final mixture is placed into molds of desired structural configuration and allowed to harden and moist cure. One configuration envisions panels or blocks to be employed singly or in built-up structures as a bullet trap that traps all or most bullets impacting it and that minimizes the chance of ricocheting bullets and release of heavy metals from the structure.
Uses of construction materials produced from a preferred embodiment of the present invention include: bullet traps and shoot houses for live-fire training; facing for berms at small-arms ranges; table tops for soldering operations using lead solder; blocks for lining trenches in training exercise areas; and use in 3D targets or manikins with a thin paper-mxc3xa2chxc3xa9 like composition having one or more layers with a coating of a material made from a preferred embodiment of the present invention.